Bacillus anthracis (causes the disease anthrax)
Life > Eubacteria >
Phylum: Firmicutes > Class: "Bacilli" > Order: Bacillales > Family:
Bacillus species form resistant spores that last for ages
Bacillus species are rod-shaped bacteria that are able to turn into resistant
spores when conditions get tough (such as when the host dies). Only one spore is
formed from each cell. Spores are highly resistant to environmental extremes and
can survive in natural environments for decades, being mainly found in the soil.
As a result it is impossible to
eradicate anthrax from regions where it has had a long history of infecting
animals (see World
Anthrax data site which has a map showing the prevalence of anthrax
Herbivorous animals become infected with Bacillus
anthracis by ingesting spores on forage plants. The spores can end up on the
plants by being blown in dust from the soil or can be deposited on leaves by
flies that have been feeding on anthrax-infected carcasses. The primary route of
infection for herbivorous animals is therefore via the gut.
There are 3 main ways people can become infected
- Through the skin (cutaneous). More than 95% of anthrax cases
worldwide are caused by cutaneous infection. Infection typically occurs
through people such as farmers, veterinarians, knackers and butchers, handling infected animals or animal products. The spores
enter through cuts or abrasions in the skin. After 2-3 days a small pimple
appears at the site of infection which steadily grows over the next few days
into an ulcer with a central region of black, dead tissue (called an
eschar). The ulcer is not painful unless there is a secondary infection, and
there is a build up of fluid in the tissues around it (i.e. an edema).
In most cases the disease remains limited to the initial site of infection
and eventually disappears because of the immune defenses fighting back.
However, about 20 percent of untreated cases end up in death, caused by the
bacterium and its toxins getting into the blood stream (i.e.
- Through the gut (intestinal). People can become infected with the
anthrax bacillus via the gut through eating under-cooked meat from infected
animals. Infection occurs by the bacterium entering the lining of the
intestine through a pre-existing wound and results in inflammation of the
intestine. The bacterium then spreads from the lining of the intestine to
the lymphatic system. A person infected through this intestinal route
experiences feelings of nausea, loss of appetite, vomiting and fever
followed by abdominal pain, vomiting of blood, and severe diarrhea. Death
occurs in 25% to 65% of cases of intestinal infection.
- Through the lungs (pulmonary). Infection occurs here through
breathing in of spores. Conventionally, people handling animal products such
as hides and wool, are susceptible to this form of infection because they
are likely to breath in spore-laden dust. It is for
this reason that pulmonary anthrax is commonly called wool-sorter's disease (with the
current bioterrorism attacks through the mail, mailsorter's disease might be
a more appropriate term).
Death is caused by the toxins that Bacillus anthracis produces
Bacillus anthracis cells contain plasmids
(independent circles of DNA, not part of the main genome) that produce a toxin
consisting of three different proteins.
- edema (or oedema). accumulation of fluids
in tissue spaces
- macrophages and neutrophils. Two types of
lymphocyte (white blood cells) that phagocytose or 'eat up' invading
- ATP. Adenosine 5'-triphosphate. Stores
chemical energy in the cell.
- Edema factor (EF). This protein is an
enzyme that elevates cyclic AMP in cells which in turn affects cell
membrane permeability, which is thought to result in loss of fluid
from the cells, causing edema in the host. The other effect of
EF is to get into macrophages and neutrophils and lower their ATP
reserves to the extent that their ability to consume invading
microorganisms is reduced.
- Lethal factor (LF). It is not certain how
LF operates but it is basically an enzyme that affects chemical
processes in the host cell to the extent that the cell dies.
- Protective antigen (PA). PA is not toxic
by itself but it is essential in opening the way for EF and LF to have
a toxic effect because it enables them to enter into host cells. PA is
able to bind to specific receptors on the host cell surface and in the
process it creates a binding site for either EF or LF. The complex
(either PA+LF or PA+EF) is then taken into the cell. See under
treatments, below, for information on a newly developed substance that
seems to be able to block the binding of EF or LF with PA.
Treatments available for anthrax
- Vaccinations are available for animals and
people, but the ones for people are generally reserved for those at high
risk such as those that work with animals and animal products such as skins.
The US military has been vaccinating its forces to counteract biological
their web site).
- Antibiotics are effective in killing Bacillus
anthracis but need to be given early on in the infection before the
bacillus has produced lethal amounts of toxin.
- Mourez et al. have recently developed a polyvalent
inhibitor (PVI) that stops the two toxic enzymes (EF and LF) produced by
B. anthracis from combining with the protective antigen (PA) on the
host cell surface thus preventing them from entering the host cell. This
substance has been tested successfully on rats but it is not yet available
for human use (see Nature Biotechnology 19: 958, October 2001, and a
report on this paper in Nature 413: xi, October 2001)
Why anthrax is a favoured organism in biological warfare
- Unless treated early on with antibiotics (before large
amounts of toxins are produced), it kills people.
- Large numbers of spores can be produced in the
- Spores can survive for decades in storage.
- Missiles, bombs, letters, etc can all be used to
distribute the spores.
Names for the disease anthrax
The name 'anthrax' is derived from the Greek word for coal. In medieval
times, anthrax was known as black bane. The association of the colour black
with anthrax is probably because the ulcer in the skin caused by cutaneous
anthrax often has a black central portion of dead tissue. Anthrax is also
known in English as wool-sorter's disease because people sorting wool are
particularly susceptible to the pulmonary form of infection by anthrax (see
In Afrikaans, anthrax is known as miltsiekte,
meaning spleen sickness. The spleen the main battle ground in the body where the
white blood cells attack invading organisms in the blood. Anthrax toxins become
concentrated in the regional lymph nodes and spleen and cause breakdown of the
History of the disease
- Anthrax has affected humans throughout recorded history. The 5th and
6th plagues of Egypt that are described in the book of Exodus are
thought to have been anthrax.
- Bacillus anthracis is famous in the history of bacteriology because
it was the first bacterium that was shown to cause a disease. Through
a series of rigorous experiments, the German physician Robert Koch
(born 1843, died 1910) discovered the link between this bacterium and the
disease anthrax and published his findings in 1876 (As an aside, it is
interesting to note that Robert Koch worked in Kimberley, South Africa from
1896-97 developing a vaccine against rinderpest. From 1903-1904 he also
investigated an outbreak of East Coast Fever in what was Rhodesia, now
- Louis Pasteur (born 1822, died 1895) and his co-workers
independently discovered the link between the bacterium and the disease and
also found that the spores in dead animals that had died of anthrax
survived after the animals had been buried, and were brought to the surface
- An effective animal vaccine was developed in the 1930's
by M. Sterne at the Veterinary Research Institute, Onderstepoort, South
- Only in 1954 was it realised that B. anthracis produces toxins
and that these are the main reason the host feels sick or dies.
- An effective human vaccine was developed in the mid-1960's.
Prescott, L.M, Harley, J.P. & Klein, D.A.
1999. Microbiology. 4th edition. McGraw-Hill, Boston.